1. Field of the Invention
The present invention relates to a display panel using a light-emitting element as a sub-pixel.
2. Description of the Related Art
As described in Jpn. Pat. Appln. KOKAI Publication No. 8-330600, an organic electroluminescent element serving as a light-emitting element has a layered structure in which an anode, electroluminescent layer (to be referred to as an EL layer hereinafter), and cathode are stacked on a substrate in this order. When a voltage is applied between the anode and cathode, holes and electrons are injected in the EL layer so electroluminescence occurs in the EL layer. An electroluminescent element whose substrate and substrate-side electrodes are designed to be optically transparent so that light from the EL layer exits from the substrate with the EL layer is called a bottom emission type. On the other hand, an electroluminescent element designed to output light from the EL layer from the opposite side of the substrate with the EL layer is called a top emission type.
In a display panel of active matrix driving type, one or a plurality of thin-film transistors are provided per 1-dot sub-pixel. The thin-film transistors cause an organic electroluminescent element to emit light. In a display panel described in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 8-330600, two thin-film transistors are provided for each sub-pixel. In manufacturing the display panel of active matrix driving type, a transistor array substrate is prepared by patterning thin-film transistors for each sub-pixel. After that, an organic electroluminescent element is patterned on the surface of the transistor array substrate in correspondence with each sub-pixel. The organic electroluminescent elements are patterned after the thin-film transistors because the temperature in patterning the thin-film transistors is higher than the heatproof temperature of the organic electroluminescent elements.
The thin-film transistors are patterned for each sub-pixel. Hence, the plurality of organic electroluminescent elements are patterned in a matrix such that the lower electrode (e.g., the anode) to be connected to the thin-film transistors is formed independently for each sub-pixel. On the other hand, the counter electrode (e.g., the cathode) is formed on the entire surface as a common electrode shared by all organic electroluminescent elements.
In the above-described apparatus, the EL layers may be damaged by thermal/chemical factors during formation of the counter electrode. To suppress damage to the EL layers, the formation time of the counter electrode is shortened as much as possible. However, when the formation time of the counter electrode is short, the counter electrode becomes thin. When organic electroluminescent elements have the top emission structure, the counter electrode is preferably made as thin as possible such that attenuation of light emitted from the EL layers during passing through the counter electrode is minimized as much as possible.
However, when the counter electrode is made thin, the sheet resistance of the counter electrode increases. When the resistance of the counter electrode is high, the voltage of the counter electrode cannot become uniform in a plane. Hence, the voltage level difference becomes conspicuous in a plane. More specifically, since the counter electrode is formed on the entire surface as a common electrode, the light emission intensity varies between the organic electroluminescent elements even if a voltage of predetermined level is applied to all sub-pixel electrodes. For this reason, the light emission intensity is not uniform in a plane.